This invention relates generally to glass compositions and more particularly to low compressibility, low expansivity glass compositions for use with optical fibers.
Typical optical fibers have a core of material with a refractive index higher than the refractive index of the surrounding cladding material, thus permitting light propagation by internal reflection. The materials used to produce the core and cladding are glass compositions designed to provide the proper refractive indexes to permit light propagation in the fiber.
In addition, the optical fiber is generally coated with one or more substrate layers of material to protect the optical fiber from the environment. These substrate coatings are typically moisture proof plastic, metal, or rubber materials which have mechanical strength and chemical durability. For example, U.S. Pat. No. 4,213,672 to Autlich et al. discloses a single and double layer plastic substrate coating.
Prior methods and coating materials have, however, offered little protection from fluctuations in the transmitted signals caused by certain variations in the external environment. Optical path variations caused by external pressure and temperature fluctuations can interfere with long distance signal transmissions, particularly with optical fibers used with acoustic sensors at significant ocean depths.
Temperature sensitivity can be an important consideration in optical fiber design. In communication systems, changes in phase can deteriorate the signal in high-bandwidth communications. Temperature-induced phase shifts add to the noise in fiber sensors, e.g. acoustic, magnetic, and gyroscopic. Similar problems exist with fiber-optic temperature sensors in that the sensing fiber should be temperature sensitive, but the optical fiber lead and reference fiber should not be. Attempts to eliminate the effects of temperature on optical fibers have generally involved matching the glass compositions of the core, cladding, and substrate so as to prevent interference by temperature fluctuations. U.S. Pat. No. 4,482,205 to Lagakos et al. discloses an optical fiber made temperature insensitive by matching the Young's modulus, expansion coefficient and Poission ratio of the fiber components. Another example of this work is Lagakos et al. "Temperature-Induced Optical Phase Shifts in Fibers" App. Optics 20:13, p. 2305-8, July 1, 1981, which discusses maximizing the temperature sensitivity of the fibers.
More recent attempts to eliminate the effects of pressure on optical fibers have involved adding extra protective layers on top of the glass fiber. U.S. Pat. No. 4,427,263 to Lagakos et al. discloses an optical fiber waveguide rendered pressure insensitive by concentrically surrounding it with a substrate of high bulk modulus material having a wall of selected thickness whereby acoustic pressures on the waveguide induce phase change cancellations.
Prior methods have not yielded a substrate coating material which effectively eliminates the interference caused by fluctuations in temperature and pressure. A protective composition which can produce optical fibers insensitive to these pressure and temperature fluctuations is needed to minimize the interference and to maximize the signal to noise ratio in the transmitted signal.